Method and apparatus of interference alignment in cellular network

ABSTRACT

Disclosed are a method and an apparatus of interference alignment in a cellular network. The method of interference alignment in the cellular network includes: receiving, by a base station, an improper signal from a terminal; and decoding, by the base station, the improper signal based on an improper decoding vector, wherein the improper signal is a signal generated by only a modulation symbol corresponding to a real number value, the improper decoding vector is determined based on an improper precoding vector, and the improper precoding vector has only the real value and separates a real number space and an imaginary number space of a received signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(e), this application claims the benefit ofU.S. Provisional Patent Application Ser. No. 61/862,498, filed on Aug.5, 2013, the contents of which are hereby incorporated by referenceherein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method and an apparatus of interference alignment ina cellular network.

2. Related Art

In recent years, a service requiring ultra-high wireless communicationsuch as wireless Internet has increased rapidly. As a result, a researchinto a communication technique that can the ultra-high wirelesscommunication for a next-generation wireless communication system hasbeen actively progressed. Therefore, a lot of researches for a pluralityof users to efficiently use resources including time, a frequency, aspace, and the like for communication were progressed. However, there isa problem that a high channel capacity for ultra-high communicationcannot be acquired due inter-user interference when users are more thangiven resources by techniques such as a frequency division accesstechnique, a time-division access technique, a code division accesstechnique, and the like in the related art.

Accordingly, in recent years, in order to solve the problem that thehigh channel capacity cannot be acquired due to the inter-userinterference, an interference alignment (IA) technology that separates adesired signal and into different spaces is proposed, and as a result,theoretical development of a transmission and reception techniqueundesired interference has been reformed. It is demonstrated that such atechnique prevents performance degradation by interference withoutcomplicated error correction encoding under a multiple user environmentof a general interference channel, acquire a degree-of-freedom bymaximizing the use of the given resources, and acquire the high channelcapacity.

In detail, the interference alignment technique can almost a channelcapacity of an interference channel under a situation in which asignal-to-ratio is very high. The interference alignment technology isextended to the cellular network as well as the interference channel tobe researched. It is revealed that in the cellular network constitutedby two cells, users that are positioned in other cell are applied to abase station of a current cell may allow interference signals to bealigned and received into a specific signal space, and as a result, alot of signal spaces for users in the current cell may be ensured. Forexample, when respective users transmits one stream, the users may allowan interference signal applied to the base station of other cell to bealigned and received into one-dimension signal space.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method ofinterference alignment in a cellular network.

Another object of the present invention is to provide an apparatus thatperforms a method of interference alignment in a cellular network.

In accordance with an embodiment of the present invention, a method ofinterference alignment in a cellular network includes: receiving, by abase station, an improper signal from a terminal; and decoding, by thebase station, the improper signal based on an improper decoding vector,wherein the improper signal is a signal generated by only a modulationsymbol corresponding to a real number value, the improper decodingvector is determined based on an improper precoding vector, and theimproper precoding vector has only the real value and separates a realnumber space and an imaginary number space of a received signal.

In accordance with another embodiment of the present invention, a basestation that performs interference alignment in a cellular network,includes: a radio frequency (RF) unit implemented to transmit or receivea radio signal; and a processor selectively connected to the RF unit,wherein the processor is implemented to receive an improper signal froma terminal and decode the improper signal based on an improper decodingvector, the improper signal is a signal generated by only a modulationsymbol corresponding to a real number value, the improper decodingvector is determined based on an improper precoding vector, and theimproper precoding vector has only the real value and separates a realnumber space and an imaginary number space of a received signal.

According to the present invention, a user can transmit and receive datawith more improved performance than the existing method in a cellularnetwork where a numerical figure is saturated by using an interferencealignment iteration algorithm based a symbol having improperness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating an interference channelaccording to an embodiment of the present invention.

FIG. 2 is a conceptual diagram illustrating a cellular network accordingto an embodiment of the present invention.

FIG. 3 is a flowchart illustrating a method for determining an improperprecoding vector and an improper decoding vector according to anembodiment of the present invention.

FIG. 4 is a flowchart illustrating a method for determining an improperprecoding vector and an improper decoding vector according to anembodiment of the present invention.

FIG. 5 is a graph illustrating a result of simulating interferencealignment based on an improper symbol according to an embodiment of thepresent invention.

FIG. 6 is a block diagram illustrating a wireless communication systemaccording to an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A terminal (user equipment, UE) may be fixed or movable and may becalled other terms such as a mobile station (MS), a mobile terminal(MT), a user terminal (UT), a subscriber station (SS), a wirelessdevice, a personal digital assistant (PDA), a wireless modem, a handhelddevice, and the like.

The base station generally represents a fixed station that communicateswith a terminal, and may be called different terms such as anevolved-NodeB (eNB), a base transceiver system (BTS), an access point,and the like.

In recent years, services and users requiring ultra-high wirelesscommunication such as wireless Internet has increased rapidly. Inparticular, in a floating population center of a downtown area, moreusers than a capacity supported by a communication system exist, and asa result, it is difficult to satisfy individual quality of service. Inorder to satisfy a quality of service requested by a plurality ofterminals, interference alignment technology is researched.

The interference alignment technology is the technology that separatelytransmits a desired signal and undesired interference to differentspaces. When the interference alignment technology is used, performancedegradation by interference under a multiple user environment may beprevented and simultaneously a maximum degree-of-freedom may be acquiredby maximizing the use of given resources. Accordingly, by using theinterference alignment technology, a high channel capacity may beacquired. However, interference alignment technologies known up to nowin a wireless communication network have problems to be described below.

In the case of the existing interference alignment technology, only aninterference channel is considered and a direct channel is notconsidered. The direct channel indicates a channel through which theterminal (or mobile station (MS) transmits a signal to a base station(or base station) to which the terminal belongs in the case of an uplinkand indicates a channel through which the base station (BS) transmits asignal to the mobile station (MS) included in the BS in the case of adownlink. Since a main purpose of an interference network interferencealignment is an alignment of interference as well as a cellular networkinterference alignment, only the interference channel is considered andthe direct channel is not considered, in general.

However, the interference alignment method may not be optimal in aninfinite (SNR) section but a finite SNR section. If the direct channelis to be considered in the interference alignment method, a gain may beincreased at the time of receiving a desired signal in the terminal orthe base station. Therefore, when the direct is to be considered in theinterference alignment method, relatively higher sum-rate may beachieved as compared with the case in which the direct channel is notconsidered. When the direct channel is considered in the interferencealignment method, global channel information may be required.

In recent years, in a lot of interference alignment methods, a specificnode having the global channel information is present and the specificnode generally notifies a transmission method for interference alignmentto each terminal or terminal However, it may be actually difficult forthe specific node to know the global channel information.

Distributed interference alignment (IA) is a representative algorithm tosolve an interference alignment problem in the existing interferencechannel. A distributed interference alignment method in the interferencechannel representatively includes a signal to interference-plus-noiseratio (Max-SINR) algorithm, and a minimum mean square error (MMSE)-IA,and an iterative algorithm similar thereto. The distributed interferencealignment method may be performed based on reciprocity of a channel. Thechannel reciprocity may be generally established in an environment usingtime-division duplexing (TDD).

When the channel reciprocity is established, a direction of a receivedbeam set for minimum interference in a receiver becomes a direction inwhich minimum interference is applied to other transmitters at the timewhen the receiver performs transmission contrary to this.

However, in a network in a saturated state, it is difficult to separatethe desired signal and the undesired interference signal due to shortageof a signal space due to a plurality of terminals. Accordingly,satisfactory performance may not be achieved by only the existingMax-SINR and the existing MMSE-IA algorithm. Accordingly, in theembodiment of the present invention, a method is disclosed, whicharbitrarily ensures the signal space capable of using the distributed IAwhen the signal space is short in a saturation state in the number ofusers in a cell.

A probability variable (alternatively, random variable) in which areciprocal autocorrelation coefficient is 0 may be proper or a properprobability variable. On the contrary, a probability variable in whichthe reciprocal autocorrelation coefficient is not 0 may be improper oran improper probability variable. In general, a symbol used forcommunication is a proper probability variable having a complex value.That is, the symbol used of the communication has power of a real numberpart and power of an imaginary number part and has a characteristic inwhich a mutual correlation coefficient is 0. However, in the cellularnetwork in the saturated state, it is necessary to ensure the signalspace in order to acquire desired interference alignment performance dueto the shortage of the signal space. In order to solve the problem, itis necessary to ensure the signal space by using improperness of atransmission symbol.

That is, in the interference alignment method according to theembodiment of the present invention in order to ensure the performanceof the interference alignment in a cellular network in which usernumerical figures are saturated by considering the improper symbol, thesignal space may be ensured by using more improved performance than theexisting method.

FIG. 1 is a conceptual diagram illustrating an interference channelaccording to an embodiment of the present invention.

In FIG. 1, an interference channel in communication between Ktransmitters 300-1, 300-2, . . . , 300-k and K receivers 350-1, 350-2, .. . , 350-k is disclosed.

Referring to FIG. 1, K transmitters 300-1, 300-2, . . . , 300-k and Kreceivers 350-1, 350-2, . . . , 350-k exist and the respectivetransmitters may transfer signals to all of K receivers 350-1, 350-2, .. . , 350 -k. For example, based on a specific receiver (for example, afirst receiver 350-1), only a signal from a specific transmitter (forexample, a first transmitter 300-1) may be a desired signal transmittedthrough a direct channel and signals transmitted from remainingtransmitters (for example, a second transmitter 300-2 to a k-thtransmitter 300-k) may be interference signals transmitted through theinterference channel.

In the case of an interference alignment algorithm of the existinginterference channel, the interference alignment may be performed basedon channel reciprocity. The channel reciprocity may be generallyestablished in a time-division duplexing (TDD) based communicationenvironment. In the embodiment of the present invention, the channelreciprocity is extended to the cellular network to assume an equivalentinterference channel environment.

FIG. 2 is a conceptual diagram illustrating a cellular network accordingto an embodiment of the present invention.

Referring to FIG. 2, a cellular uplink channel is extended to bemodified to the equivalent interference channel.

In a method of interference alignment according to an embodiment of thepresent invention, distributed interference alignment may be performedbased on the channel reciprocity with respect to the equivalentinterference channel modified by extending the cellular uplink channel.

Equation 1 below represents modeling that the base station receivesuplink data.

$\begin{matrix}{y^{b} = {{\sum\limits_{l = 1}^{B}\; {\sum\limits_{k = 1}^{K}\; {H_{lk}^{b}v_{lk}x_{lk}}}} + w^{b}}} & {\text{<}{Equation}\mspace{14mu} 1\text{>}}\end{matrix}$

Respective variables of Equation may have meanings described below.

y^(b) representing a signal vector received by a b-th base stationH_(lk) ^(b) representing a channel when a k-th user terminal in a 1-stcell sends to a b-th base station

v_(lk) representing a precoding vector of the k-th user terminal in the1-th cell

x_(lk) representing a transmission symbol of the k-th user terminal inthe 1-th cell w^(b) representing noise of a b-th base station

B and K may be determined based on the number of base stations and thenumber of user terminals.

In the embodiment of the present invention, in order to ensure thesignal space by extending a signal to interference-plus-noise ratio(Max-SINR) algorithm and a minimum mean square estimation-interferencealignment (IA) algorithm based on improperness of a symbol, Equation 1which is an uplink receiving signal model modeled on the assumption ofthe existing properness may be extended like Equation 2.

$\begin{matrix}{{{\overset{\_}{y}}_{b} = {{\sum\limits_{l = 1}^{B}\; {\sum\limits_{k = 1}^{K}\; {{\overset{\_}{H}}_{lk}^{b}{\overset{\_}{v}}_{lk}x_{lk}}}} + {\overset{\_}{w}}^{b}}}{{\overset{\_}{y}}^{b}\overset{\Delta}{=}\begin{bmatrix}{{Rc}\left\{ y^{h} \right\}} \\{{Jm}\left\{ y^{b} \right\}}\end{bmatrix}}{{\overset{¨}{H}}_{lk}^{b}\overset{\Delta}{=}\begin{bmatrix}{{Rc}\left\{ H_{lk}^{b} \right\}} & {{- {Jm}}\left\{ H_{lk}^{b} \right\}} \\{{Jm}\left\{ H_{lk}^{b} \right\}} & {{Rc}\left\{ H_{lk}^{b} \right\}}\end{bmatrix}}{{\overset{¨}{w}}_{b}\overset{\Delta}{=}\begin{bmatrix}{{Rc}\left\{ w^{b} \right\}} \\{{Jm}\left\{ w^{b} \right\}}\end{bmatrix}}{{\overset{¨}{v}}_{lk}\overset{\Delta}{=}\begin{bmatrix}{{Rc}\left\{ v_{lk} \right\}} \\{{Jm}\left\{ v_{lk} \right\}}\end{bmatrix}}} & {\text{<}{Equation}\mspace{14mu} 2\text{>}}\end{matrix}$

According to the embodiment of the present invention, the variables areextended to a vector generated by joining a real number part and animaginary number part to be converted to a vector of which all valuesare only real number values as described above. Further, thecommunication based the improper symbol may be performed by only aprecoding vector design by restricting a user's transmission symbolx_(lk) to be sent to only a real number value.

When the communication based on the existing proper symbol is performed,communication is performed based on a precoding vector having onlyproperness and a complex symbol having properness. When thecommunication is performed based on the existing proper symbol, a newsignal space using a real number value and an imaginary number valuehaving improperness and a correlation of both values may not be used. Onthe contrary, as described in the embodiment of the present invention,when an improper precoding vector having only the real number valueacquired in Equation 2 and an improper symbol (for example, areal-number symbol) are used, an optimal real number value and anoptimal imaginary number value and a correlation of both values may beused according to circumstances. In particular, a short signal space maybe finely divided and used under a saturated cellular network situation.

Hereinafter, in the embodiment of the present invention, a method isdisclosed, which determines a precoding vector v_(lk), havingimproperness and a decoding vector g_(bk) having improperness by usingan algorithm to maximize an SINR by using the vector having only thereal value.

Further, hereinafter, in the embodiment of the present invention, amethod is disclosed, which determines the precoding vector v_(lk) havingimproperness and the decoding vector g_(bk) having improperness by usingan algorithm to minimize an MMSE by using the vector having only thereal value.

Equation 3 presents a method that determines the precoding vector v_(lk)having improperness and the decoding vector g_(bk) having impropernessby using the algorithm to maximize the SINR.

$\begin{matrix}{{{\overset{\_}{g}}_{bk} = {{\alpha_{k}\left( {{\sum\limits_{i = 1}^{K}\; {{\overset{\_}{H}}_{bi}^{b}{\overset{\_}{v}}_{bi}{\overset{\_}{v}}_{bi}^{T}{\overset{\_}{H}}_{bi}^{bT}}} + {\overset{\_}{R}}_{ici}^{b} + {\sigma^{2}I_{2N}}} \right)}^{- 1}{\overset{\_}{H}}_{bk}^{b}{\overset{\_}{v}}_{bk}}}{{\overset{\_}{R}}_{ici}^{b}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}\; {\sum\limits_{i = 1}^{K}\; {{\overset{\_}{H}}_{li}^{b}{\overset{\_}{v}}_{H}{\overset{\_}{v}}_{ii}^{T}{\overset{\_}{H}}_{ii}^{bT}}}}}{{\overset{¨}{v}}_{bk} = {{\beta_{k}\left( {{\overset{¨}{R}}_{ici}^{l^{\prime}} + {\sigma^{2}I_{2M}}} \right)}^{- 1}{\overset{¨}{H}}_{bk}^{bT}{\overset{\ldots}{g}}_{bk}}}{{\overset{\_}{R}}_{ici}^{l^{\prime}}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}\; {\sum\limits_{i = 1}^{K}\; {{\overset{\_}{H}}_{bk}^{lT}{\overset{¨}{g}}_{li}{\overset{¨}{g}}_{li}^{T}{\overset{\_}{H}}_{bk}^{t}}}}}} & {\text{<}{Equation}\mspace{14mu} 3\text{>}}\end{matrix}$

g _(bk) represents a decoding vector of the k-th user terminal of theb-th base station.

As described above, H _(yz) ^(x) may represent a transmission channelfunction when a z-th user in a y-th cell separated into a real numberspace and an imaginary number space transmits data to an x-th basestation and v _(xy) may be an improper precoding vector of a y-th userin an x-th cell separated into the real number space and the imaginarynumber space.

When iteration is performed through the algorithm of Equation 3, theprecoding vector having improperness may be determined When a signalspace is not sufficient, the improper precoding vector and the improperdecoding vector may be determined through the algorithm of FIG. 3. Theimproper precoding vector and the improper decoding vector may moreefficiently perform interference alignment through an effect to avoidinterference by separating signal spaces of real number and imaginarynumber parts.

In Equation 3, α_(k)=1/∥ g _(bk) ²∥ β_(k)1/∥ v _(bk) ²∥ representconversion constants for making the size of the vector to 1. σ²represents a distribution of noise.

FIG. 3 is a flowchart illustrating a method for determining an improperprecoding vector and an improper decoding vector according to anembodiment of the present invention.

FIG. 3 discloses the method that determines the precoding vector v_(lk)having the improperness and the decoding vector g_(bk) having theimproperness by using the algorithm to maximize the SINR.

Referring to FIG. 3, the precoding vector is initialized (step S300).

The precoding vector may be initialized to v ₁, . . . v _(K).

The decoding vector is updated (step S310).

The decoding vector may be updated through

${\overset{\_}{g}}_{bk} = {\left( {{\sum\limits_{l = 1}^{B}\; {\sum\limits_{i = 1}^{K}\; {{\overset{\_}{H}}_{li}^{b}{\overset{\_}{v}}_{li}{\overset{\_}{v}}_{bi}^{T}{\overset{\_}{H}}_{li}^{bT}}}} + {\sigma^{2}I_{2N}}} \right)^{- 1}{\overset{\_}{H}}_{bk}^{b}{\overset{\_}{v}}_{bk}}$

of Equation 3 described above.

The size of the decoding vector is adjusted (step S320).

The size of the decoding vector updated through step S310 may beadjusted based on ∥ g _(bk) ²∥=1.

The precoding vector is updated (step S330).

The precoding vector may be updated through

${\overset{\_}{v}}_{bk} = {\left( {{\sum\limits_{l \neq b}^{B}\; {\sum\limits_{i = 1}^{K}\; {{\overset{\_}{H}}_{li}^{bT}{\overset{\_}{g}}_{li}{\overset{\_}{g}}_{bi}^{T}{\overset{\_}{H}}_{li}^{b}}}} + {\sigma^{2}I_{2N}}} \right)^{- 1}{\overset{\_}{H}}_{bk}^{bT}{\overset{\_}{g}}_{bk}}$

of Equation 3 described above.

The size of the updated precoding vector is adjusted (step S340).

The size of the precoding vector updated through step S330 may beadjusted based on ∥ v _(bk) ²∥=1.

After the size is adjusted, the process returns to step S310 again toupdate the decoding vector based the updated precoding vector.

Equation 4 below presents a method that determines the precoding vectorv_(lk) having improperness and the decoding vector g_(bk) havingimproperness by using the algorithm to minimize the MMSE.

$\begin{matrix}{{{{\overset{¨}{g}}_{bk} = {\left( {{\sum\limits_{i = 1}^{K}\; {{\overset{¨}{H}}_{bi}^{b}{\overset{¨}{v}}_{bi}{\overset{\ldots}{v}}_{bi}^{T}{\overset{¨}{H}}_{bi}^{bT}}} + {\overset{¨}{R}}_{ici}^{b} + {\sigma^{2}I_{2N}}} \right)^{- 1}{\overset{¨}{H}}_{bk}^{b}{\overset{¨}{v}}_{bk}}}{\overset{¨}{R}}_{ici}^{b}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}\; {\sum\limits_{i = 1}^{K}\; {{\overset{¨}{H}}_{li}^{b}{\overset{¨}{v}}_{li}{\overset{¨}{v}}_{li}^{T}{\overset{¨}{H}}_{li}^{bT}}}}}{{\overset{¨}{v}}_{bk} = {\left( {{\overset{\_}{R}}_{ici}^{l^{\prime}} + {\lambda_{bk}I_{2M}}} \right)^{- 1}{\overset{\_}{H}}_{bk}^{bT}{\overset{\ldots}{g}}_{bk}}}{{\overset{\_}{R}}_{ici}^{l^{\prime}}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}\; {\sum\limits_{i = 1}^{K}\; {{\overset{\_}{H}}_{bk}^{lT}{\overset{\_}{g}}_{li}{\overset{\_}{g}}_{li}^{T}{\overset{\_}{H}}_{bk}^{l}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In this equation, λ_(bk) represents a value used to make the size of theprecoding vector to 1.

FIG. 4 is a flowchart illustrating a method for determining an improperprecoding vector and an improper decoding vector according to anembodiment of the present invention.

FIG. 4 discloses the method that determines the precoding vector v_(lk)having the improperness and the decoding vector ghk having theimproperness by using the algorithm to minimize the MMSE.

Referring to FIG. 4, the precoding vector is initialized (step S400).

The precoding vector may be initialized to v ₁, . . . v _(K)..

The decoding vector is updated (step S410).

The decoding vector may be updated through

${\overset{\_}{g}}_{bk} = {\left( {{\sum\limits_{l = 1}^{B}{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{li}^{b}{\overset{\_}{\upsilon}}_{li}{\overset{\_}{\upsilon}}_{bi}^{T}{\overset{\_}{H}}_{li}^{b\; T}}}} + {\sigma^{2}I_{2N}}} \right)^{- 1}{\overset{\_}{H}}_{bk}^{b}{\overset{\_}{\upsilon}}_{bk}}$

of Equation 4 described above.

The precoding vector is updated (step S420).

The precoding vector may be updated through

${\overset{\_}{\upsilon}}_{bk} = {\left( {{\sum\limits_{l \neq b}^{B}{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{li}^{bT}{\overset{\_}{g}}_{li}{\overset{\_}{g}}_{bi}^{T}{\overset{\_}{H}}_{li}^{b}}}} + {\lambda_{bk}I_{2N}}} \right)^{- 1}{\overset{\_}{H}}_{bk}^{bT}{\overset{\_}{g}}_{bk}}$

of Equation 4 described above.

In step S420, λ_(bk) satisfying ∥ v _(bk) ²∥=1 may be selected whileupdating the precoding vector.

Step S410 is again performed to update the decoding vector based on theprecoding vector updated through step S420.

FIG. 5 is a graph illustrating a result of simulating interferencealignment based on an improper symbol according to an embodiment of thepresent invention.

In FIG. 5, a simulation result in the case of 3 cells, 3 users per cell,4 transmitting antennas, and 2 receiving antennas is illustrated.

Under an environment in which a space for interference alignment isshort, a result of performing the interference alignment based on animproper symbol according to the embodiment of the present invention anda case of performing the interference alignment based on an propersymbol in the related art are compared with h each other.

Referring to FIG. 5, in the case of performing the interferencealignment based on the improper symbol according to the embodiment ofthe present invention, it may be seen that sum-rate is increased as anSNR is increased by comparing the case of performing the interferencealignment based on the improper symbol according to the embodiment ofthe present invention and the case of performing the interferencealignment based on the proper symbol with each other.

FIG. 6 is a block diagram illustrating a wireless communication systemaccording to an embodiment of the present invention.

Referring to FIG. 6, a base station 600 includes a processor 610, amemory 620, and a radio frequency (RF) unit 630. The memory 620 isconnected with the processor 610 to store various pieces of informationfor driving the processor 610. The RF unit 620 is connected with theprocessor 910 to transmit and/or receive the radio signal. The processor610 implements a function, a process, and/or a method which areproposed. In the aforementioned embodiment, the operation of the basestation may be implemented by the processor 610.

Similarly, a UE 650 includes a processor 660, a memory 670, and an RFunit 680. The memory 670 is connected with the processor 660 to storevarious pieces of information for driving the processor 660. The RF unit680 is connected with the processor 660 to transmit and/or receive theradio signal. The processor 660 implements a function, a process, and/ora method which are proposed. In the aforementioned embodiment, theoperation of the terminal may be implemented by the processor 660.

For example, the processors 610 and 660 may perform the interferencealignment based on an improper signal in a cellular network. Theprocessors 610 and 660 are implemented to receive the improper signalfrom the terminal and decode the improper signal based on the improperdecoding vector, in which the improper signal may be a signal generatedby only a modulation symbol corresponding to a real number value, theimproper decoding vector may be determined based on the improperprecoding vector, the improper precoding vector may have only the realnumber value, and a real number space and an imaginary number space of areceived signal may be separated.

The processor may include an application-specific integrated circuit(ASIC), another chip set, a logic circuit and/or a data processingapparatus. The memory may include a read-only memory (ROM), a randomaccess memory (RAM), a flash memory, a memory card, a storage medium,and/or another storage device. The RF unit may include a basebandcircuit for processing the radio signal. When the embodiment isimplemented by software, the aforementioned technique may be implementedby a module (a process, a function, and the like) that performs theaforementioned function. The module may be stored in the memory andexecuted by the processor. The memory may be present inside or outsidethe processor and may be connected with the processor through variouswell-known means.

In the aforementioned exemplary system, methods have been describedbased on flowcharts as a series of steps or blocks, but the methods arenot limited to the order of the steps of the present invention and anystep may occur in a step or an order different from or simultaneously asthe aforementioned step or order. Further, it can be appreciated bythose skilled in the art that steps shown in the flowcharts are notexclusive and other steps may be included or one or more steps do notinfluence the scope of the present invention and may be deleted.

What is claimed is:
 1. A method of interference alignment in a cellularnetwork, comprising: receiving, by a base station, an improper signalfrom a terminal; and decoding, by the base station, the improper signalbased on an improper decoding vector, wherein the improper signal is asignal generated by only a modulation symbol corresponding to a realnumber value, the improper decoding vector is determined based on animproper precoding vector, and the improper precoding vector has onlythe real value and separates a real number space and an imaginary numberspace of a received signal.
 2. The method of claim 1, wherein: theimproper decoding vector is${{\overset{\_}{g}}_{bk} = {{\alpha_{k}\left( {{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{bi}^{b}{\overset{\_}{\upsilon}}_{bi}{\overset{\_}{\upsilon}}_{bi}^{T}{\overset{\_}{H}}_{bi}^{b\; T}}} + {\overset{\_}{R}}_{ici}^{b} + {\sigma^{2}I_{2N}}} \right)}^{- 1}{\overset{\_}{H}}_{bk}^{b}{\overset{\_}{\upsilon}}_{bk}}},$the R _(ici) ^(b) is${{\overset{\_}{R}}_{ici}^{b}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{li}^{b}{\overset{\_}{\upsilon}}_{li}{\overset{\_}{\upsilon}}_{li}^{T}{\overset{\_}{H}}_{li}^{bT}}}}},$the v _(bk) as the improper precoding vector is${{\overset{\_}{\upsilon}}_{bk} = {{\beta_{k}\left( {{\overset{\_}{R}}_{ici}^{l^{\prime}} + {\sigma^{2}I_{2M}}} \right)}^{- 1}{\overset{\_}{H}}_{bk}^{bT}{\overset{\_}{g}}_{bk}}},$the R_(ici) ^(l′) is${{\overset{\_}{R}}_{ici}^{l^{\prime}}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{bk}^{lT}{\overset{\_}{g}}_{li}{\overset{\_}{g}}_{li}^{T}{\overset{\_}{H}}_{bk}^{l}}}}},$the H _(yz) ^(x) represent a transmission channel function when a z-thuser in a y-th cell separated into a real number space and an imaginarynumber space transmits data to an x-th base station, the v _(xy) is s animproper precoding vector of a y-th user in an x-th cell separated intothe real number space and the imaginary number space, and the α_(k)=1/∥g _(bk) ²∥ and the β_(k)=1/∥ v _(bk) ²∥ represent conversion constantsfor making the size of the vector to
 1. 3. The method of claim 1,wherein: the improper decoding vector is${{\overset{\_}{g}}_{bk} = {\left( {{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{bi}^{b}{\overset{\_}{\upsilon}}_{bi}{\overset{\_}{\upsilon}}_{bi}^{T}{\overset{\_}{H}}_{bi}^{b\; T}}} + {\overset{\_}{R}}_{ici}^{b} + {\sigma^{2}I_{2N}}} \right)^{- 1}{\overset{\_}{H}}_{bk}^{b}{\overset{\_}{\upsilon}}_{bk}}},$the R _(ici) ^(b) is${{\overset{\_}{R}}_{ici}^{b}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{li}^{b}{\overset{\_}{\upsilon}}_{li}{\overset{\_}{\upsilon}}_{li}^{T}{\overset{\_}{H}}_{li}^{bT}}}}},$the v _(bk) as the improper precoding vector is V _(bk)=( R _(ici)^(l′)+λ_(bk)I_(2M))⁻¹ H _(bk) ^(bT) g _(bk), the R _(ici) ^(l′) is${{\overset{\_}{R}}_{ici}^{l^{\prime}}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{bk}^{lT}{\overset{\_}{g}}_{li}{\overset{\_}{g}}_{li}^{T}{\overset{\_}{H}}_{bk}^{l}}}}},$the H _(yz) ^(x) represent a transmission channel function when a z-thuser in a y-th cell separated into a real number space and an imaginarynumber space transmits data to an x-th base station, the v _(xy) is animproper precoding vector of a y-th user in an x-th cell separated intothe real number space and the imaginary number space, and the λ_(bk) isa value for making the size of the improper precoding vector to
 1. 4. Abase station that performs interference alignment in a cellular network,comprising: a radio frequency (RF) unit implemented to transmit orreceive a radio signal; and a processor selectively connected to the RFunit, wherein the processor is implemented to receive an improper signalfrom a terminal and decode the improper signal based on an improperdecoding vector, the improper signal is a signal generated by only amodulation symbol corresponding to a real number value, the improperdecoding vector is determined based on an improper precoding vector, andthe improper precoding vector has only the real value and separates areal number space and an imaginary number space of a received signal. 5.The base station of claim 4, wherein: the improper decoding vector is${{\overset{\_}{g}}_{bk} = {{\alpha_{k}\left( {{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{bi}^{b}{\overset{\_}{\upsilon}}_{bi}{\overset{\_}{\upsilon}}_{bi}^{T}{\overset{\_}{H}}_{bi}^{b\; T}}} + {\overset{\_}{R}}_{ici}^{b} + {\sigma^{2}I_{2N}}} \right)}^{- 1}{\overset{\_}{H}}_{bk}^{b}{\overset{\_}{\upsilon}}_{bk}}},$the R _(ici) ^(b) is${{\overset{\_}{R}}_{ici}^{b}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{li}^{b}{\overset{\_}{\upsilon}}_{li}{\overset{\_}{\upsilon}}_{li}^{T}{\overset{\_}{H}}_{li}^{bT}}}}},$v_(bk) as the improper precoding vector is v _(bk)=β_(k)( R _(ici)^(l′)+σ²I_(2M))⁻¹ H _(bk) ^(bT) g _(bk), the R _(ici) ^(l′) is${{\overset{\_}{R}}_{ici}^{l^{\prime}}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{bk}^{lT}{\overset{\_}{g}}_{li}{\overset{\_}{g}}_{li}^{T}{\overset{\_}{H}}_{bk}^{l}}}}},$the H _(yz) ^(x) represent a transmission channel function when a z-thuser in a y-th cell separated into a real number space and an imaginarynumber space transmits data to an x-th base station, the v _(xy) is animproper precoding vector of a y-th user in an x-th cell separated intothe real number space and the imaginary number space, the α_(k)=1/∥ g_(bk) ²∥ and the β_(k)=1/∥ v _(bk) ²∥ are conversion constants formaking the size of the vector to 1, and the σ² represents a distributionof noise.
 6. The base station of claim 4, wherein: the improper decodingvector is${{\overset{\_}{g}}_{bk} = {\left( {{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{bi}^{b}{\overset{\_}{\upsilon}}_{bi}{\overset{\_}{\upsilon}}_{bi}^{T}{\overset{\_}{H}}_{bi}^{b\; T}}} + {\overset{\_}{R}}_{ici}^{b} + {\sigma^{2}I_{2N}}} \right)^{- 1}{\overset{\_}{H}}_{bk}^{b}{\overset{\_}{\upsilon}}_{bk}}},$the R _(ici) ^(b) is${{\overset{\_}{R}}_{ici}^{b}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{li}^{b}{\overset{\_}{\upsilon}}_{li}{\overset{\_}{\upsilon}}_{li}^{T}{\overset{\_}{H}}_{li}^{bT}}}}},$the v _(bk) as the improper precoding vector is V _(bk)=( R _(ici)^(l′)+λ_(bk)I_(2M))⁻¹ H _(bk) ^(bT) g _(bk), the R _(ici) ^(l′) is${{\overset{\_}{R}}_{ici}^{l^{\prime}}\overset{\Delta}{=}{\sum\limits_{l \neq b}^{B}{\sum\limits_{i = 1}^{K}{{\overset{\_}{H}}_{bk}^{lT}{\overset{\_}{g}}_{li}{\overset{\_}{g}}_{li}^{T}{\overset{\_}{H}}_{bk}^{l}}}}},$the H _(yz) ^(x) represent a transmission channel function when a z-thuser in a y-th cell separated into a real number space and an imaginarynumber space transmits data to an x-th base station, the v _(xy) is s animproper precoding vector of a y-th user in an x-th cell separated intothe real number space and the imaginary number space, and the λ_(bk) isa value for making the size of the improper precoding vector to 1.